1. Field of the Invention
This invention is related to the field of eddy current sensors. More particularly, this invention is related to an apparatus and method for measuring various parameters of moving, electrically conductive objects such as turbine blades, fan blades, and impeller, if necessary, through a casing, housing, or other barrier.
2. Description of the Problem
Eddy current sensors are known and widely used in a variety of applications to measure characteristics of moving, electrically conductive objects. A common use of eddy current sensors is in fans and turbines, where the sensors are used to measure parameters related to blade status. Such parameters can include detection of blade passing or stalling, measurement of tip clearance for individual blades, and observation of bending, torsion, vibration, cracks, and foreign object impact. The sensors generally work by creating a magnetic field through which the electrically conductive, nonmagnetic blades pass, thereby inducing an eddy current in the blades. The eddy current causes there to be a change in the magnetic field as the blade passes by the sensor, and a voltage is induced in a coil wound around a magnetically permeable core. The resulting electrical current pulses in the coil can be interpreted based on their size and shape to describe the blade characteristics.
There are two types of known eddy current sensors. The first, and most common, uses an alternating excitation current to generate an alternating magnetic field. The second uses a permanent magnet or magnets to generate a static magnetic field. Most often the magnets are shaped to form legs and resemble either a U or E, with the legs joined by a transverse flux bridge, creating a circuitous path for the magnetic field. In alternating magnetic field sensors, such as that shown in U.S. Pat. No. 5,942,893 to Terpay, the direction of the windings is selected to produce the desired relative field directions. In static field sensors, the orientation of permanent magnet poles determines the field directions, as shown in U.S. Pat. No. 3,932,813 to Gallant.
The blades to be measured are typically enclosed in a casing. Known magnetic sensor designs are either placed inside the casing or within holes in the casing so that the magnetic field does not have to pass through the casing in order to interact with the blades. Holes in the casing are undesirable because of resulting degraded mechanical performance, which is an increasing concern when there are multiple adjacent sensors as required to monitor blade vibration. Placing the sensor inside the casing is undesirable because of the expense of special modifications that must be made to create a recessed area outside of the blade path to accommodate the sensor. Both locations expose the sensors to an abusive environment that can include high temperature, corrosive gases, vibration, and blade contact. Internal sensors generate noisy signals and are expensive to build. Alternating current driven magnetic fields do not go through conductive casing materials at the high frequencies used and require substantial electronics and power to excite them and to filter and process the noisy signals generated in order to extract measurements. Such fields also generally lack the range required to function adequately outside a casing.
Known sensors, therefore, are generally inadequate to be mounted external to casings. Alternating magnetic fields do not penetrate the casing as well as static fields do. Known static field sensors, however, have inadequate strength and inappropriate geometry to generate an adequate magnetic field to penetrate casings. For example, the sensor disclosed in U.S. Pat. No. 3,932,813 is shown mounted inside a fan housing and has an E-shaped magnet with the poles quite close to each other. Its sensing coils are opposed and produce a difference signal in order to cancel noise and DC offset. This geometry limits that sensor's range severely and would be inappropriate for an external sensor.
Accordingly, there is a need to avoid the expense of internal eddy current sensors and the associated environmental and performance problems, while generating an adequate magnetic field to penetrate a casing and interact with the blades or other moving, electrically conductive parts.